Method of electrodepositing zinc



United States Patent Off ce e METHOD OF ELECTRODEPOSITING ZlNC JohnB..Winters, Westlake, Ohio, assignor to Ann F. Hull and Adrian Medert,Cleveland, Ohio, as cotrustees No Drawing. Application September 22,1954,

- Serial No. 457,797

6 Claims. (Cl. 204-55) This invention relates to the electrodepositionof zinc, and is more particularly directed to a new and improved bathcomposition and process for producing bright zinc deposits wherein thecyanide bath contains in solution bath-soluble reaction products ofepichlorhydrin'with ammonia or primary amines. i

Zinc plated from cyanide baths is widely employed as a coating toprotect steel base metals from corn):

sion and to provide corrosion resistant undercoats for organic and otherprotective finishes which are applied thereover. A zinc coating upon anarticle plated from a cyanide plating bath is dull gray or dark incolor, relatively crystalline in structure, is poor in appearance,stains readily and accordingly such coating has a very limited field ofapplication. It has long been common practice to obtain a coating ofimproved physical characteristics, particularly in appearance, byincorporating in the cyanide plating bath various organic and inorganicsubstances which are generally referred to as addi tion agents orbrighteners.

Heretofore it has been the common practice in obtaining bright zincdeposits from cyanide plating baths to use as addition agents acombination of organic and inorganic compounds, and optimum' resultsdepended on maintaining a pre-determined balance between the'concentrations of the organic and inorganic constituents. Accordinglythe prior practice required relatively close control over a plurality ofthe constituents of the plat ing bath by chemical analysis, or othermeans if analytical methods were not feasible. In the commercialoperation of plating baths employing such addition agents, numerousdrawbacks were encountered by reason of such addition agents which werein addition to those brought about by any unbalanced relationshipbetween the organic and inorganic substances. For example, one of thedisadvantages of the combination of organic-inorganic addition agents isthat the inorganic constituent is a soluble metal compound, the metal ofwhich codeposits with the zinc, greatly to the detriment of some of itsphysical properties, such as ductility and corrosion resistance. Suchco-deposition causes stains and undesirable appearance of the coatedsurface upon the subsequent application of surface treatments which arecommonly used to passivate the zinc surface or to produce thereon atarnish or corrosion resistant film. The purer the zinc deposit upon thesurface of the article the better it is for corrosion protection and foraccepting subsequent surface treatments designed to produce passive orprotective films such as the chromate type 'or phosphate coatings topromote lacquer adhesion.

it is among the objects of my invention to provide an addition agent forzinc cyanide plating bath compositions by means "of which bright, smoothand relatively pure Zinc deposits can be obtained therefrom. A furtherobject of my invention is to provide zinc plating baths and a platingprocess employing the same which produce bright zinc deposits that areuniformly responsive to 2,791,554 Patented May 7, 1.957

. H. z oxidizing bright dips, chromate type protective dips andphosphatizing dips.

7. :These objects are accomplished briefly by the use of aqueous cyanidezinc baths that contain bath-soluble reaction products of epichlorhydrinwith ammonia or with primary amines either alone or for some purposes incombination with an aromatic aldehyde selected from the group consistingof'oxyheterocyclic aldehydes, phenyl ethers and methoxy benzaldehydes.Such reaction products may be known as polyepoxyamines and will :be sodesignated hereafter.

While my novel brighteners are effective in any commercially operablezinc cyanide plating bath, best results are obtained with theapproximate bath composition and operating conditions shown in Table 1,which can be varied as required for special purposes in accordance withthe known skill in the art.

Table 1 Bath composition: Grams per liter Zinc cyanid 60 Sodium cyanide42 Sodium hydroxide v V I Ratio i l la.Cl l (free 22d combmed =2]Operating conditions:

Temperature-about F. Cathode current density--. 20 amps. per sq. ft.Anodes Zinc'of suitable purity.

The plating bath is prepared by dissolving the constituents in water,after which the bath should be purified by adding about one-half gram ofsodium sulphide per liter of'ba-th dissolved in water, or the bath maybe treated by stirring about'two grams of zinc 'dust per liter of bathfor a few hours, then settling and decanting or filtering.

=1 have found that addition agents used for cyanide zinc plating bathsin accordance with the prior art are subject to certain disadvantages.For example, theme of aromatic aldehydes requires very frequentadditions because of their instability in the bath, and the samedisadvantage applies to colloids such as gelatin. The use of polyvinylalcohol is quite common, but this material is subject to thedisadvantages of salting out of the bath, forming an undesirable scum onthebath surface, producing an undesirable yellow color to the brightzinc deposit, and furthermore is not uniform in its composition.

The above disadvantages are entirely overcome with my new and novelbath-soluble polyepoxyamines. These polyepoxamines are unusually stablein the plating bath, their solubility is more than sufiicient topreclude their being salted out, they do not produce yellowdeposits upona plated article and they can be readily produced in batches that areuniform in effect when used in a plating bath. The addition ofbath-soluble polyepoxaminesalso greatly improves the covering power ofcyanide zinc plating baths to the extent that recesses in the articlewhich cannot be covered withthe' metal'under' ordinary operatingconditions are readily covered if such addition agents are present. e r

Fihe polyepoxamines may be'prepar'ed by any one of a number of wellknown and established procedures. However, for both maximumeffectiveness and bathstability, the preparation of the polyepoxamine inaccordance with the following procedure has been found most desirable. ii

Using a jacketed stainless steel reaction vessel, 10 gallons of waterand 56 pounds of aqueous ammonia (29%) are combined and cooled to atemperature commensuratewith theability of the apparatus to convey awayheat. I have found that a temperature of from 6'to 10 degrees is themost desirable. Five gallons of epichlorhydrin are then added to thevessel. Cold water circulated within the jacket serves to convey awaythe heat of reaction and to maintain the substances within the vesselwithin a desirable temperature range. An initial, very slow reaction orcondensation takes place which leads to an intermediate product. This islater polymerized at a controlled rate. The evidence of such a slowreaction is continuous liberation of heat and a continuous drop in pHfrom about 12.5 to 8.0 as determined by a glass electrode.

After the initial reaction has been completed which takes approximatelya period of three hours, polymerization is promoted by gradual elevationof the temperature to about 101 degrees centigrade or the refluxtemperature. After maintaining the high temperature for about threeadditional hours, the desired degree of polymerization is reached andthe resulting polyepoxyamine is suitable for addition to zinc cyanideplating baths. The product made in accordance with this procedurepossesses a relatively low degree of polymerization which is mostefiective in the zinc plating bath within the range of current densitiesfrom about 5 to 125 amperes per square foot.

Another procedure for preparing polyepoxyamines suitable for use as, anadditive for zinc plating baths consists in adding epichlorhydrin slowlyto the diluted ammonia at higher temperatures. Using the same type ofreaction vessel, 10 gallons of water and 56 pounds of aqueous ammonia(29%) are combined and heated to about 60 degrees centigrade.Epichlorhydrin in the amount of 5 gallons is then slowly added to thevessel with the continued agitation of the mixture therein whilemaintaining a constant temperature by means of the circulation of coldwater in the jacket until all of the epichlorhydrin has been added. Thespeed at which the epichlorhydrin is added is determined by ability ofthe circulating water through the jacket of the container to convey awaythe reaction heat. After the initial exothermic reaction is complete asevidenced by no further tendency toward temperature rise for a period offrom 30 to 60 minutes, hot water under pressure is circulated throughthe jacket to slowly elevate the temperature to 101 degrees centigradeor reflux temperature as in the first procedure. The

resultant product shows a higher degree of polymerization than the firstproduct and is particularly elfective in the zinc plating bath in thelower current density ranges of from 2 to 40 amperes per square foot.

In each of the above procedures for preparing a polyepoxyamine in lieuof aqueous ammonia a solution of 32 pounds of ethylene diamine in 15gallons of water may be used. The same amount of epichlorhydrin is used.In this case it is desirable to remove any excess of ethylene diaminewhich may be present by distillation prior to final polymerization.

The amount or concentration of my novel addition agents will depend uponvarious conditions such as bath composition, degree of brightness ofdeposit desired and degree of covering power. A desirable range ofeffectiveness has been found through the use of from 0.25 to 4.0 gramsper liter of cyanide zinc plating bath. The preferred range ofconcentration is from 0.5 to 2 grams per liter of cyanide zinc platingbath.

Zinc cyanide plating baths containing my novel brighteners producebright zinc deposits direct from the plating baths. The zinc isrelatively pure as it contains no codeposited brightener metal and itsnormal ductility has not been materially impaired. If under someconditions of operation a light yellow film occurs over the bright zincsurface, such film can be removed by dipping in any of the commerciallyused bright dips such as dilute nitric acid (approximately /4%), diluteacidified hydrogen peroxide, etc., all of which are well known in theart, with out staining the bright zinc surface.

Bright zinc deposits from my novel bath composition 2,791,554 A I, H

can be subjected to any of the many chromate type dips for producingpassive surfaces or protective coatings with uniform and satisfactoryresults. Likewise such bright zinc deposits will take a uniformphosphatizing treat ment by any of the widely used commercial processesfor producing undercoats on zinc for organic finishes and the like.

My novel brighteners greatly improve the plating characteristics of thezinc cyanide bath in which they are used by increasing both its coveringpower and throwing power, and by providing a broad range of currentdensities over which bright deposits can be plated. While my novelbrighteners are self-sufficient in producing bright, smooth, relativelypure zinc electrodeposits from zinc cyanide plating baths containingthem in solution, there are certain conditions encountered, for example,in barrel plating, where the use of an aromatic aldehyde such as anisicaldehyde or heliotropin, in conjunction with my novel brightenersproduces superior results mainly from the standpoint of coverage on lowcurrent density areas with improvement in brightness thereon.Accordingly, under such conditions I find it advantageous to add about,1 to 1 gram of such aromatic aldehyde per liter of zinc cyanide bathcontaining my novel brightener. Experiments have proven that the actionof my novel brighteners with such compounds is synergistic as identicalresults can not be obtained by the use of either alone. Similarly, ifdesired for some applications, a metal brightening agent such as acompound of molybdenum or chromium or others given in United StatesPatent No. 2,080,520, may be employed with my polyepoxyamines.

Having thus described my invention so that those skilled in the art mayunderstand and practice the same, what I desire by Letters Patent isembodied in the appended claims.

I claim:

1. An aqueous zinc cyanide electroplating bath con taining a bathsoluble polyepoxyamine resulting from the condensation reaction ofepichlorhydrin with a primary amine in sufficient amount to provide abright zinc deposit.

2. An aqueous zinc cyanide electroplating bath containing from 0.25 to4.0 grams per liter of a bath-soluble polyepoxyamine resulting from thecondensation reaction of epichlorhydrin with a primary amine.

3. In the process for electrodepositing zinc, the step comprisingdepositing zinc from an aqueous zinc cyanide bath containing in solutionfrom 0.25 to 4.0 grams per liter of bath of a bath solublepolyepoxyamine resulting from the condensation reaction ofepichlorhydiin with a primary amine.

4. In the process for electrodepositing zinc, the step comprisingdepositing zinc from an aqueous zinc cyanide bat-h containing insolution from 0.25 to 4.0 grams per liter of a bath solublepolyepoxyamine resulting from the condensation reaction ofepichlorhydrin with a primary amine and an oxyheterocyclic aldehyde.

5. In the process for electrodepositing zinc, the step comprisingdepositing zinc from an aqueous zinc cyanide bath containing in solutionfrom 0.25 to 4.0 grams per liter of a bath soluble polyepoxyamineresulting from the condensation reaction of epichlorhydrin with aprimary amine and a phenyl ether.

6. In the process for electrodepositing zinc, the step comprisingdepositing zinc from an aqueous zinc cyanide bath containing in solutionfrom 0.25 to 4.0 grams per liter of a bath soluble polyepoxyamineresulting from the condensation reaction of epichlorhydrin with aprimary amine and a methoxy benzaldchyde.

Hoffman Dec. 28, 1948 Ellis Dec. 15, 1953

1. AN AQUEOUS ZINC CYANIDE ELECTROPLATING BATH CONTAINING A BATH SOUBLEPOLYEPOXYAMINE RESULTING FROM THE CONDENSATION REACTION OFEPICHLORHYDRIN WITH A PRIMARY AMINE IN SUFFICIENT AMOUNT TO PROVIDE ABRIGHT ZINC DEPOSIT.